1. Field of the Invention
The present invention relates an image forming apparatus employing an electro-photographic process such as a laser printer, an electro-photographic copying machine and the like, and particularly, to an image forming apparatus having a toner recycle mechanism, and a controlling method therefor.
2. Related Art Statement
In an Image forming apparatus 1 employing an electro-photographic process such as a laser printer, an electrophotographic copying machine and the like, as shown in FIG. 1, first, the surface of a photosensitive drum 2 is charged by a charging device 3 to a fixed surface potential, then the surface of the photosensitive drum 2 is exposed by an exposing device 4 such as a semiconductor laser, and the surface potential is decayed to form an electrostatic latent image. And, a bias voltage is applied to the surface of a developing roller 6 of a developing device 5, toner charged within the developing device 5 is adhered to an electrostatic latent image forming section on the surface of the photosensitive drum 2, developed and actualized to form a toner image on the surface of the photosensitive drum 2.
On the other hand, paper P is carried to a transfer region by carrying rollers 7, 7, the paper P is charged by a transfer device 8, and a toner image formed on the surface of the photosensitive drum 2 is transferred to the surface of the paper P. The paper P is stripped from the surface of the photosensitive drum 2 by a stripping device 9, a toner image is fixed to the surface of the paper P by a fixing device (not shown), and the paper P is discharged outside the device by a discharging roller (not shown).
After the toner image has been transferred to the surface of the paper P, toner remained on the surface of the photosensitive drum 2 is scraped off by a blade 11 of a cleaning device 10 to recover it, and then a load remained on the surface of the photosensitive drum 2 is removed by an eliminator 12 to place the surface of the photosensitive drum 2 in an initial state.
In a developing device 5 employing a two-components developing system, as shown in FIG. 1, a developer D comprising a carrier C as magnetic powder and a toner T as colored resin powder is encased in the developing device 5, the carrier C and the toner T are mixed and stirred by stirring screws 13, 14, and the toner is charged by friction. When the toner T is consumed by development, toner TF is supplied from a toner hopper 15 into the developing device 5, and the supplied toner TF is mixed with the carrier C, stirred and charged, in a manner similar to that described above.
A toner recycle mechanism 16 is constituted by a carrying screw 17 disposed within the cleaning device 10, a toner carrying tube 18 for communications between the cleaning device 10 and the developing device 5, a carrying screw 19 disposed within the toner carrying tube 18, and the like. Toner TR recovered into the cleaning device 10 is introduced into a toner carrying tube 18 by the carrying screw 17, and carried into the toner carrying tube 18 by a carrying screw 19, and the toner TR is supplied into the developing device 5.
When the developing device 5 is continued to be driven for a long period of time, toner T being mixed and stirred becomes deteriorated, and a degree of agglomeration of toner T within the developing device 5 changes also. When a degree of agglomeration of toner T within the developing device 5 becomes excessively low, a quantity of toner adhered to the surface of the photosensitive drum 2 increases, and therefore a fog phenomenon occurs in which fine toner is adhered to the whole paper P. On the other hand, when a degree of agglomeration of toner T within the developing device 5 becomes excessively high, a quantity of toner adhered to the surface of the photosensitive drum 2 decreases, and therefore an unevenness in density occurs in a toner image formed on paper P.
In view of the foregoing, there has been proposed an image forming apparatus wherein in order that, even if a degree of agglomeration of toner T within the developing device 5 is changed, an image which is excellent in quality and stabilized in density can be formed on paper P, a degree of agglomeration of toner T is detected on the basis of rotational torque of the stirring screws 13, 14, and a development bias voltage is controlled corresponding to the degree of agglomeration of toner T (see Japanese Patent Application Laid-Open No. Hei 11-352758 Publication).
However, in the image forming apparatus having a toner recycle mechanism sometimes involves a case where a large quantity of recycle toner TR recovered is supplied into the developing device 5, and the rotational torque of the stirring screws 13, 14 elevates. In such a case as described, as disclosed in Japanese Patent Application Laid-Open No. Hei 11-352758 Publication, even if a development bias voltage is made high, a phenomenon such as fog in paper P or scattering of toner is not improved because an amount of charge of toner T present within the developing device 5 (fresh toner TF and recycle toner TR) is uneven.
That is, it is contemplated that dominant causes that a degree of agglomeration of toner T within the developing device 5 changes are both 1) a deterioration of toner T, and 2) an excessively large supply of recycle toner TR). However, the fact that by which dominant cause the degree of agglomeration of toner T within the developing device 5 was changed cannot be judged merely by detecting the rotational torque of the stirring screws 13, 14. Therefore, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. Hei 11-352758 Publication, the phenomenon such as fog in paper P or scattering of toner could not be improved positively.